Iron is an essential factor for the growth of E. coli K12 and the tonB gene product is an essential component of all high affinity iron transport systems in E. coli. The tonB gene encodes a 36,000 dalton membrane protein that appears to be involved in membrane energy transduction. The physical location of the tonB structural gene and its promoter have been determined by analysis of a series of tonB transducing phages and recombinant tonB plasmids. Preliminary studies suggest that tonB gene expression is induced by iron limitation. The object of the proposed work is to determine the structure of the tonB gene, the precise cellular location of its gene product, and the mechanism(s) by which tonB gene expression is regulated. The DNA sequence of the tonB regulatory region and structural gene will be determined and the sites of tonB transcription and translation will be defined. TonB-lacZ gene fusions will be used to examine the regulation of tonB expression in greater detail. This research will provide insight into the regulation and synthesis of bacterial membrane proteins, and it will lead to a better understanding of the role that the tonB gene product plays in iron transport. Present awareness that the course of bacteremic infection is determined, in part, by the availability of iron in the blood and the ability of pathogenic organisms to sequester iron, also lends medical relevance to these studies.